Bipolar transistors, and in particular heterojunction bipolar transistors, (HBTs), have become the components of choice in power amplification applications that amplify radio frequency (RF) signals. These bipolar transistors, such as HBTs are typically built using either silicon (Si) and/or III-V semiconductor technologies. HBTs exhibit good response characteristics for both low-power and high-power applications. Furthermore, HBTs have also demonstrated great potential over a wide range of operating frequencies. Bipolar transistors, and in particular HBTs, are also strong candidates to meet the requirements of cellular handsets as well as 802.11a and 802.11b WLAN standards which power amplification of RF signals between 2.4 GHz and 5.8 GHz or greater.
HBTs may be arranged in stacks, often referred to as transistor cells, in order to amplify a RF input signal. Many power amplification devices have multiple transistor cells that are operably associated with one another in order to provide power amplification. The advantages of bipolar transistors, and in particular HBTs, have made these arrangements a popular choice for power amplification devices associated with RF transceivers. Unfortunately, one of the fundamental concerns when utilizing a stack of bipolar transistors is the prevention of thermal runaway. Thermal runaway is often caused by what may be referred to as current hogging by one or more transistors in the transistor cell. If one of the bipolar transistors (or even a small region of one of the bipolar transistors) in the stack runs even slightly hotter than the other transistors, the current in the slightly hotter bipolar transistor will locally increase. In turn, this causes the slightly hotter bipolar transistor to run even hotter than the other bipolar transistors thereby further increasing the current hogging of the slightly hotter bipolar transistor. This positive feedback thermal effect may continue until the hotter bipolar transistor or transistor region is destroyed. For example, metal regions used to form the contacts of the bipolar transistors can be melted away as a result of the excessive temperatures resulting from thermal runaway. Consequently, the transistor cell may no longer be capable of operating and sometimes the entire multi-cell power amplification device may no longer be capable of operating due to the failure of the single (or multiple) bipolar transistor(s). To prevent thermal runaway, a ballasted resistance is often coupled to the base of the bipolar transistors, the emitters of the bipolar transistors, or both to the base and the emitters of the bipolar transistors. This provides the necessary negative feedback to prevent slightly hotter bipolar transistors from current hogging.
Another important aspect of designing a reliable RF power amplification device is the management of the heat dissipation from the transistor cells during normal operation. Since the conversion of DC to RF power in an power amplification device can vary by approximately 10 to 70%, a large amount of DC power ends up being converted into heat. This heat needs to be efficiently dissipated. Otherwise, the semiconductor substrate may be heated outside a temperature range for reliable operation. Additionally, the heat may cause damage to the components of the power amplification device, such as the metallic regions used to form the contacts of the bipolar transistors.
To mount the power amplification device in an integrated circuit (IC) package assembly, a conventional mounting application mounts a backside of the power amplification device to a large metal region in a board containing a thermal heat sink for the power amplification device. The conventional approach works well with ballast resistors, as the heat generated locally by the bipolar transistors is dissipated through the backside of the power amplification device and through the heat sink of the board. While the conventional approach does a fairly good job of dissipating heat, conventional mounting applications also typically involve wire bonding in order to form the connections to the power amplification device. The stray inductances and mutual inductances created between the wires of wire bonded power amplification devices can cause significant cross talk and signal isolation problems. Accordingly, other mounting techniques such as different bump technologies and copper pillar technologies can be used to mount the power amplification devices on an IC package assembly. These mounting technologies provide better isolation for the connections to the power amplification device.
Unfortunately, the dissipation of heat generated from the bipolar transistors is complicated by these types of mounting technologies. For example, when the power amplification device is mounted using a bumped die technology, the semiconductor substrate of the power amplification device is typically upside-down and thus may not face the heat spreader region. In this case, the heat dissipation of the power cells is now very different from the conventional application, as the heat flow now needs to dissipate vertically from the semiconductor substrate through the solder bumps and, ultimately to the board. A problem therefore arises when the power amplification device needs to employ ballast resistors, such as emitter ballast resistors, and a mounting technology such as bumped die technology. In essence, the heat flow from the emitter region is strongly compromised as heat and electrical current must dissipate horizontally to reach the emitter ballast resistor. Only after this horizontal heat dissipation through the emitter ballast resistor can the heat then flow vertically and be collected by the solder bumps. This added lateral heat flow is very inefficient, as the lateral thermal resistance is fairly high and will cause a significant increase in the operating temperature of the bipolar transistors, potentially creating very unsafe and unreliable operating conditions.
In addition, another problem resulting from using mounting technologies such as bump die technologies and copper pillar technologies is that the design for the ballast resistors is typically spatially inefficient. These resistors or resistances must often be provided by components that are horizontally distal to the transistor cell. These horizontally distal resistive components also need to be provided on a semiconductor substrate which may not be a good thermal conductor. As a result, not only do these horizontally distal resistive components cause heat dissipation inefficiencies where heat generated by the transistor cell is concentrated at the horizontally distal resistive components, but, also, these configurations for the ballast resistors results in the consumption of space.
Thus, what is needed is an arrangement for a power amplification device that allows the power amplification device to be mounted using bump technologies and copper pillar technologies while providing both better spatial efficiency and better heat dissipation.